1. Field of the Invention
This invention relates generally to the field of wireless communications and, more particularly, to satellite antenna feed systems.
2. Description of the Related Art
Satellite communications systems convey information on carrier signals in a number of frequency bands approved for this purpose by regulatory organizations and standards bodies. Among the most widely-used bands are the C-band (3.625-4.200 GHz for downlink, 5.850-6.425 GHz for uplink), X-band (7.250-8.400 GHz), and Ku-band (10.950 to 12.750 GHz for downlink, 14.000-14.500 GHz for uplink).
Turning now to FIG. 1, a typical satellite communications system is shown. For uplink transmission from a ground station 110 to a satellite 109, a data signal (which may be digital or analog) is first sent to a modulator circuit 112 in ground station 110. There, the data signal modulates a carrier signal with a frequency in one of the permitted frequency bands. The modulated carrier signal is then sent to an input port on an antenna feed 102. Antenna feed 102 is typically a waveguide assembly positioned such that its radiated output is efficiently coupled to a system of one or more reflector units 100. Antenna feed 102 acts as a transducer that converts the modulated carrier signal into radiated electromagnetic waves 114 that illuminate the reflector units. The waves are then directed by reflector units 100 to satellite 109.
Downlink transmission from satellite 109 to ground station 110 is commonly received by the same antenna system used for the uplink transmission. The above process is reversed for the antenna system to receive a signal from satellite 109. A modulated carrier transmitted by satellite 109 is first directed by reflectors 100 into antenna feed 102. Antenna feed 102 then acts as a transducer to route the received waves to transceiver 108. A demodulator receives the modulated carrier signal from the receive ports and recovers the data stream transmitted by satellite 109.
One potential limitation with prior art systems is that in order to transmit/receive in all three of the popular frequency bands C, X and Ku, three physically separate antenna feed structures are typically needed. For example, a C-band antenna feed with its own I/O port may be needed for transmitting/receiving in the C-band; an X-band antenna feed with its own I/O port may be needed for transmitting/receiving in the X-band; and a Ku-band antenna feed with its own I/O port may be needed for transmitting/receiving in the Ku-band.
Since three separate antenna feed structures are typically needed, it follows that the data transmission/reception from one parabolic reflector can usually occur only in one frequency band at a time. For example, before data transmission/reception can occur in the C-band, the C-band antenna feed may often be physically moved such that its I/O port is located at the focal point of the parabolic reflector. Then, to switch data transmission/reception to the X-band, the C-band antenna feed is physically moved out of the focal point of the reflector so that the X-band antenna feed can be physically moved to the focal point of the reflector. Consequently, the number of data streams which are transmitted/received simultaneously may be limited to the number of data streams which fit into one frequency band. Having to physically move the C-band, X-band and Ku-band antenna feed structures to and from the focal point of the reflector is a time-consuming and tedious operation. Furthermore, if the movement is not done accurately, misalignment problems between the reflector and the I/O port of the antenna feed structure may occur.
For example, when the I/O port of an antenna feed is misaligned with its reflector, the radiation pattern of the transmitted electromagnetic waves may become distorted. This distortion may in turn interfere with transmissions from other independent sources. Consequently, many ground stations limit their transmissions/receptions to just one of the three bands C, X, and Ku.
It is common to use antennas having paraboloidal reflectors (e.g., reflector 100 in FIG. 1) in applications such as space communications where radio frequency signals in the form of microwave frequency electromagnetic waves are transmitted between an earth station and a satellite or vice versa. Such antennas may be constructed in a prime focus configuration where microwave frequency energy is coupled to a transceiver by an antenna feed mounted near a focal point of the paraboloidal reflector. The antennas may also be constructed in other configurations such as Gregorian or Cassegrain. Doubly-shaped reflectors may be used as well. These configurations use a small hyberboloidal subreflector mounted near the focal point of the paraboloidal reflector, allowing the feed to be placed between the paraboloidal and hyperboloidal reflectors. Paraboloidal reflector antennas are also used in radar and other communications applications as well.
Regardless of the feed configuration or system application, it is the purpose of the feed to connect a transceiver to the paraboloidal reflector. Antennas intended for operation over multiple frequency bands may normally require a corresponding number of multiple feeds and subreflectors. U.S. Pat. No. 4,092,648 to Fletcher, et al., issued May 30, 1978, and assigned to the National Aeronautics and Space Administration of the United States Government, incorporated herein by reference in its entirety, shows a typical multiple-band antenna having a main reflector that diverts energy to a subreflector and then to a flange. The flange is arranged to pass radiation in a first frequency band to a first horn. Energy in a second frequency band is reflected by the flange to an auxiliary reflector. The auxiliary reflector is arranged to feed energy to a second horn.
If operation in more than two frequency bands is required, subreflector, auxiliary reflector, and multiple horn configurations may become more complicated. In some instances, it may be desirable to tilt and rotate the subreflectors about a symmetry axis in order to provide better tracking of the satellite or other signal source. This further complicates construction and operation of the antenna. It is typically desirable to keep the antenna assembly as small and simple as possible.
Various communication systems employ more than one frequency band for electromagnetic signals radiated from a transmitting station to receiving station. An important example of such a communication system is a satellite communication system wherein various bands of signals are transmitted between a satellite above the earth (synchronous orbit) and ground stations on the earth. As previously noted, three such bands of interest are the C band, X band, and Ku band, which together extend over two octaves of the communication frequency spectrum. Within each of the bands, there is frequency space allocated for reception of signals at the satellite and for transmission of signals from the satellite. The C band itself extends over approximately an octave, operates at both linear and circular polarizations, and includes a receive sub-band in the range of 3.625-4.200 GHz and a transmit sub-band in the range of 5.850-6.425 GHz. The X band includes a receive sub-band in the range of 7.250-7.750 GHz (gigahertz), and a transmit sub-band for transmission from the satellite in the range of 7.900-8.400 GHz. The Ku band operates at both linear and circular polarizations, and includes a receive sub-band from 10.950 to 12.750 GHz, and a transmit sub-band of 14.000-14.500 GHz. Collectively, these frequency bands extend over approximately two octaves of the communications spectrum.
Historically, it has been the practice to provide separate antennas for transmission or reception on each of the bands because there is insufficient bandwidth on any one of the antenna systems or terminals to transmit more than one of the bands. In some cases, where bands are close together and, collectively, do not occupy an excessive amount of spectral space, it has been possible to share a plurality of bands on one antenna. However, basically separate antennas have typically been employed for different portions of the spectrum. In particular, there is no adequate single-point antenna feed system which can cover plural octave bandwidths which include the C, X, and Ku bands.
A further problem arises in the case of satellite communication transportable earth stations in that there is a need to minimize the weight of the system. The use of numerous antennas for communication at various frequency bands may defeat this purpose. In addition, it is advantageous to employ a common phase center for all frequencies of radiation transmitted from the earth station and received at the earth station. This is typically lacking when several antenna feeds are mounted at different times upon an earth terminal. Furthermore, changing the feed system for each frequency band and refocusing the feed requires extra time and trained personnel. The same problems exist for an earth terminal at a fixed location that performs the difficult and tedious process of exchanging feeds and refocusing.
The foregoing problems are compounded by the previously described spectral utilization. The C band and the Ku band are commercial satellite bands which are spaced apart in the spectrum and, therefore, facilitate the filtering of signals in the two bands so as to permit transmission on one band without significant interference with signals on the other band. However, in some applications there is a need to employ the X band (which is a military band) in conjunction with the C band. However, due to the fact that the X band is contiguous to the C band, it is difficult to separate the two bands in a common antenna system. Presently available antenna and feed structures appear unable to accomplish this task adequately.